1. Field of the Invention
The present invention relates to a method for depositing a layer on a substrate with a sputtering process and, more particularly, this invention relates to a method for reducing the lateral component of a particle flux in a sputtering process.
2. Description of the Prior Art
Sputtering processes are often applied in semiconductor manufacturing processes for depositing layers of various materials. In these known processes, a plasma is ignited in a sputtering reactor between two electrodes. A sputter target is arranged on the cathode; a substrate wafer on which the layer is to be deposited is arranged on the anode. Positively-charged ions of an inert atmosphere such as argon are accelerated onto the target and eject individual atoms or molecules from the target when they impact the target. The target is composed of a material required for the layer deposition. The particles that are extracted form a particle flux which impacts and distributes in all directions at the target surface. There are two components of sputtered particle fluxes in sputtering processes: a vertical component that vertically impinges the substrate wafer and a lateral component that impinges at a certain angle relative to the surface of the substrate wafer.
Both components of the particle flux contribute to the generation of the layer when a layer is deposited on a substrate wafer having an essentially planar surface. When the substrate comprises structures having aspect ratios of 1 and less than 1, it is essentially only the vertical flux component that contributes to the layer generation. The aspect ratio is the ratio of diameter to depth. Aspect ratios of 1 and less than 1 generally occur when filling via holes with metallization layers. The lateral flux component impacts the side walls of the structures and for the most part covers only the side walls. Consequently, the filling of structures, for example via holes, is made more difficult.
One known solution to this problem is described in an article titled Sputtering Tools: Still on Target?, Semiconductor International, August 1992, at page 42, by Ron Iscoff. This article describes the use of a collimator placed between the substrate wafer and the target to intercept the lateral component of the particle flux before it impacts the substrate wafer. Coverage of the side walls of, for example, via holes, is thus avoided, so that the quality of the via hole filling is improved. This, however, also results in a decrease in the deposition rate. Moreover, the deposition rate is not uniformly distributed over the substrate wafer when a collimator is used in a standard sputter reactor without additional modifications. This is due to the fact that standard reactors are designed so that the sum of lateral and vertical flux components is uniform. The vertical component is not independently optimized.
Optimization of the vertical component can be achieved by varying the magnetic field at the target. This results in a vertical flux component having a uniform distribution when viewed independently whereas the lateral flux component has a relatively non-uniform distribution. The lateral flux component is intercepted by the collimator, so that its inhomogeneity does not deteriorate the deposition.
Because the correlation between the homogeneity of the deposition rate and the magnetic field is extremely indirect, optimization of the distribution of the vertical flux component in this manner requires an exhaustive series of experiments. Moreover, modification of the magnetic field is usually only practical if it is performed by the reactor manufacturer. This leads to an additional delay in the process development. Finally, the interception of the lateral component in the collimator leads to aging of the collimator. The lateral flux component creates a layer deposition on the side walls of the holes in the collimator which gradually results in the collimator holes being sputtered shut. This aging effect cannot be reduced or eliminated because the lateral flux component is non-uniform over the expanse of the collimator due to the optimization of the vertical flux component.